They're aphids and the quickest and most effective way to deal with them is to use a systemic insecticide.
This will mean that the garden wildlife such as ladybirds, butterflies and bees will not be with you in the garden though.

I use a hose on mine and or if it happens again I then use soapy water this gets rid of them and is a safe option. Make sure your plant is well watered as I find they go first to dry plants.

I was looking for advice on this problem and the comment on the plants being well watered rings true as they are all over some Nasturtiums and they are in a very sunny spot and I noticed tonight when I watered that they were v.dry. I have used an insectt friendly spray and will see what tomorrow brings. Water seems to have no effect on them so I will try soapy water if the spray has not helped. I noticed that lots of ants were on the plant and I wondered if they were feeding on the aphids.